The present invention relates to a tubular casing for foodstuffs, in particular for its use as a sausage casing, and more especially to a casing which is permeable to smoke, wherein a web of regenerated cellulose is formed into a tube by curving the web along the longitudinal axis, meeting or overlapping the edges and adhesively bonding the edges, in particular with a film strip also of regenerated cellulose. The present invention also relates to a process for the manufacture of such casings.
Tubular casings made from a web, for example of regenerated cellulose, which are manufactured by folding the web along the longitudinal axis and gluing the overlapping edge zones running parallel to the longitudinal axis are already known (U.S. Pat. Nos. 2,148,844, 2,226,442, 2,685,769, 2,685,770, 2,686,725, 2,757,495 and 2,773,773). The gluing operations hitherto described comprise either the use of solvents with which the surface of the web is incipiently dissolved and tackified, whereupon an "adhesive" is produced in situ on the surface of the web, or the use of webs which are coated with a thermoplastic resin or are composed of a thermoplastic material which can be sealed when heat and pressure are applied.
Thus, U.S. Pat. No. 2,653,432 describes a process for bonding the overlapping edges of a film by means of a strip of thermoplastic material, which is located between the overlapping edges and becomes tacky on heating. Also, the film can be composed of regenerated cellulose and can have a fiber reinforcement, if appropriate. Acrylate and polyvinylidene chloride are mentioned as examples of this thermoplastic material. This known tubular casing can also have a moisture-tight coating on the outside, the overlapping edges then being kept free of coating and the remaining surface being kept free of thermoplastic material. The casing material can be pre-stretched, preferably in the longitudinal direction.
This old process, which has been known for about 30 years, is hardly used any longer. In particular, an overlapping seam formed by sealing the superposed edge zones is not sufficiently strong and is also completely unsatisfactory for reasons of a appearance. Moreover, these known tubular casings can be manufactured only in a relatively involved manner, i.e., at high temperatures, with long residence times until the adhesive has set and, in some cases, with the aid of acids or organic solvents which can be removed from the tubular casing only with difficulty and by very expensive measures.
For sausages which must have a tubular casing which is permeable to smoke and gas, the known casings with a glued seam are not suitable, since the seam region is not sufficiently permeable.
Therefore, previously, sausage casings have been manufactured predominantly from seamless material, although some disadvantages must be accepted in the case of these seamless tubes. In particular, this process is very cost-intensive.
Seamless tubes of fiber-reinforced regenerated cellulose are usually manufactured by the following essential process steps. A web of non-woven fiber is formed to give the shape of a tube, the edges of which overlap and are glued with viscose. After one-sided or two-sided coating with viscose from annular slot dies, the tube is coagulated, regenerated to give cellulose hydrate gel and dried. During this process, the tube is supported by the gas mixture formed during the coagulation and regeneration and, during drying it is supported by an enclosed volume of air which has been introduced. If appropriate, the tube is stretched in the longitudinal and transverse directions in order to improve the mechanical properties.
Disadvantageously, weakened zones, so-called spinning edges, which run parallel to the longitudinal axis in the edge region appear, for example, on the edges of the gel tube laid flat. Moreover, the stretching process also has disadvantages.
Neither the internal pressure of the process gases nor the internal pressure during drying can be controlled with sufficient accuracy, so that the diameter of the tubes cannot be kept constant within narrow limits. A further disadvantage is that, when tubes are stretched by means of an expanding gas, the ratio of the stretching forces acting in the tangential and axial directions is fixed, so that, for this reason, the mechanical properties, in particular the shrinking properties and the strength in the longitudinal and transverse directions, of the seamless tube are also fixed.